Nano graphene platelet-based conductive inks

ABSTRACT

A nano graphene platelet-based conductive ink comprising: (a) nano graphene platelets (preferably un-oxidized or pristine graphene), and (b) a liquid medium in which the nano graphene platelets are dispersed, wherein the nano graphene platelets occupy a proportion of at least 0.001% by volume based on the total ink volume. The ink can also contain a binder or matrix material and/or a surfactant. The ink may further comprise other fillers, such as carbon nanotubes, carbon nano-fibers, metal nano particles, carbon black, conductive organic species, etc. The graphene platelets preferably have an average thickness no greater than 10 nm and more preferably no greater than 1 nm. These inks can be printed to form a range of electrically or thermally conductive components.

FIELD OF THE INVENTION

The present invention relates generally to the field of conductive inks,and more particularly to nano graphene platelet-based inks that areelectrically and thermally conductive.

BACKGROUND OF THE INVENTION

Conductive inks, particularly carbon-based inks, have been widely usedin the manufacture of conducting elements in printed circuits and sensorelectrodes. Other major markets for conductive inks include emergingapplications, such as displays, backplanes, radio frequencyidentification (RFID), photovoltaics, lighting, disposable electronics,and memory sensors, as well as traditional thick film applications inwhich screen printing is used in the creation of PCBs, automobileheaters, EMI shielding, and membrane switches. There is tremendousinterest in the field of RFID and printed electronics. This is becausemajor retailers and institutions need to be able to more accurately andefficiently track inventory, and RFID and printed electronics areconsidered the ideal solution.

Among various electrically conductive nano particles, silver is commonlyconsidered the material of choice for RFID antennas; but, nano silverparticles are very expensive. A carbon-based ink typically containsparticles of graphite, amorphous carbon, or carbon black (CB) that aresuspended in a binder/resin and a solvent. These inks are applied on asubstrate surface via a number of deposition techniques, including brushpainting, syringe application, inkjet printing, screen printing, and gasassisted spraying. The ink is allowed to dry and the resultingcarbon-coated surface, if containing a binder or matrix resin, issubjected to a curing treatment. Further, printing RFID tags is seen asthe most likely way to reduce their costs to a point where such tags canbe widely used on cost sensitive items, such as food packages. Comparedto micron-scaled particles, nano-scaled particles are more amenable toinkjet printing.

For printed electronics, all conventional carbon-based conductiveparticles have one or more shortcomings. For instance, graphiteparticles are too large in size to be inkjet printable; they easily clogup the dispensing nozzles. Carbon black is not sufficiently conductingand, hence, cannot be used alone as a conductive additive in an ink.Another class of carbon materials that can be inkjet printed is thecarbon nano-tube (CNT) [Refs. 1-4]. CNTs, although relativelyconducting, are prohibitively expensive. The production of CNTsnecessarily involves the use of heavy metal elements as catalysts thatare undesirable in many applications and must be removed. The CNTs thatcontain catalysts tend to undergo sedimentation in a dispersing liquid,which is a highly undesirable feature in a conductive ink. Further, CNTstend to aggregate together and get entangled with one another due totheir high length-to-diameter aspect ratio, making it difficult todisperse CNTs in water, organic solvents, and polymer matrices (forforming nanocomposites). The aggregation and entanglement of CNTs alsodramatically increase the viscosity of the dispersing liquid [e.g.,Refs. 5 and 6], to the extent that inkjet printing of CNT inks ispossible only when an exceedingly low CNT concentration is involved.Similarly, processing of CNT-resin nanocomposite is not possible withmelt mixing/molding (e.g., via extrusion or injection molding) when CNTloading exceeds 5% by weight [Refs. 7 and 8].

Therefore, there is a need for nano particle-containing conductive inksthat exhibit the following features: (1) the conductive additives aremuch less expensive than CNTs; (2) the inks are printable, preferablyinkjet printable using a conventional, low-cost printhead; (3) theadditives and the resulting printed elements are highly conductive,electrically and/or thermally; (4) the additives can be readilydispersed in a wide range of liquid mediums and do not form a sediment;and (5) the inks can contain a high conductive additive content so thata desired amount or thickness of conductive elements can be dispensedand deposited onto a substrate in one pass or few passes (to avoid orreduce the need for repeated printing passes or overwrites). It is ofinterest to note that high thermal conductivity is a desirable featureof an additive for microelectronic packaging applications since modernmicroelectronic devices, when in operation, are generating heat at anever increasing rate. An additive with a high thermal conductivityprovides a more efficient thermal management material.

The following is a list of references that are related to the prior art:

REFERENCES CITED

-   1. J. W. Song, “Inkjet Printing of Single-Walled Carbon Nanotubes    and Electrical Characterization of the Line Pattern,”    Nanotechnology, 19 (2008) 095702 (6 pp).-   2. T. Mustonen, et al., “Inkjet Printing of Transparent and    Conductive Patterns of Single-Walled Carbon Nanotubes and PEDOT-PSS    Composites,” Phys. Stat. Sol. (b) 244 (2007) 4336-4340.-   3. W. R. Small, et al., “Inkjet Deposition and Characterization of    Transparent Conducting Electroactive Polyaniline Composite Films    with a High Carbon Nanotube Loading Fraction,” J. Materials Chem.,    17 (2007) 43594361.-   4. T. Mustonen, et al., “Controlled Ohmic and Nonlinear Electrical    Transport in Ink-printed Single-Wall Carbon Nanotube Films,”    Physical Review, B 77 (2008) 125430 (7 pp).-   5. E. K. Hobbie and D. J. Fry, “Rheology of Concentrated Carbon    Nanotube Suspensions,” The J. of Chem. Phys., 126 (2007) 124907 (7    pp).-   6. I. A. Kinloch, S. A. Roberts, and A. H. Windle, “A Rheological    Study of Concentrated Aqueous Nanotube Dispersions,” Polymer,    43 (2002) 7483-7491.-   7. S. S. Rahatekar, et al., “Optical Microstructure and Viscosity    Enhancement for an Epoxy Resin Matrix Containing Multiwall Carbon    Nanotubes,” J. Rheol. 50 (5) (2006) 599-610.-   8. Y. Y. Huang, et al., “Dispersion Rheology of Carbon Nanotubes in    a Polymer Matrix,” Physical Review, B 73 (2006) 125422 (9 pp).-   9. E. S. Kirkor, “Conducting Inks,” U.S. Pat. No. 7,097,788 (Aug.    29, 2006).-   10. O. Matarredona, et al., “Carbon Nanotune Pastes and Method of    Use,” U.S. Pat. No. 7,279,247 (Oct. 9, 2007).-   11. J. J. Mack, et al., “Chemical Manufacture of Nanostructured    Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).-   12. M. Hirata and S. Horiuchi, “Thin-Film-Like Particles Having    Skeleton Constructed by Carbons and Isolated Films,” U.S. Pat. No.    6,596,396 (Jul. 22, 2003).-   13. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for    Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent    pending, Ser. No. 11/509,424 (Aug. 25, 2006).-   14. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of    Nano-scaled Platelets and Products,” U.S. patent pending, Ser. No.    11/526,489 (Sep. 26, 2006).-   15. B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S.    Pat. No. 7,071,258 (Jul. 04, 2006).-   16. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method    of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled    Graphene Plates,” U.S. patent pending, Ser. No. 11/800,728 (May 08,    2007).-   17. A. Yu, et al., “Graphite Nanoplatelet-Epoxy Composite Thermal    Interface Materials,” J. Physical Chem., C 111 (2007) 7565-7569.

SUMMARY OF THE INVENTION

The present invention provides a nano graphene platelet-based conductiveink comprising: (a) nano graphene platelets (NGPs) wherein each of theplatelets comprises a graphene sheet or multiple graphene sheets and theplatelets have a thickness no greater than 100 nm, and (b) a liquidmedium in which the NGPs are dispersed, wherein the NGPs occupy aproportion of at least 0.001% by volume based on the total ink volume.

An NGP is essentially composed of a sheet of graphene plane or multiplesheets of graphene plane stacked and bonded together. Each grapheneplane, also referred to as a graphene sheet or basal plane, comprises atwo-dimensional hexagonal structure of carbon atoms. Each platelet has alength and a width parallel to the graphite plane and a thicknessorthogonal to the graphite plane. In an NGP, the largest dimension isdefined as the length, the smallest dimension as the thickness, and thethird or intermediate dimension as the width. By definition, thethickness of an NGP is 100 nanometers (nm) or smaller, with asingle-sheet NGP being as thin as 0.34 nm. The length and width of a NGPare typically between 1 μm and 20 μm, but could be longer or shorter.Several methods have been developed for the production of NGPs [e.g.,Refs. 11-15].

The graphene platelets preferably occupy a proportion of at least 1% byvolume (for inkjet printability) and up to 40% by volume (for use inscreen printing or other dispensing methods) based on the total inkvolume. We have surprisingly observed that up to 60% by volume of NGPscan be easily incorporated into a liquid medium, such as water andethanol, as opposed to the commonly recognized notion that carbonnanotubes (CNTS) can only be properly dispersed in a liquid for lessthan 10% by volume.

The presently invented conductive ink is preferably inkjet printablesince inkjet printing is a cost-effective way to achieve patterns ofvarious materials on both rigid and flexible substrates. Inkjet printingof electrically conductive nano particle-based inks offer a verypractical platform for generating electrical components, such aselectrodes and interconnects. More preferably, inkjet printing isconducted using a conventional, inexpensive printhead in a commondesk-top printer. This type of printer typically requires the viscosityof the ink to be in the range of 3-30 mPa·S (centi-poise or cP). It isof significance to note that the viscosity of a CNT-based ink can not bein this useful range unless the CNT proportion is exceedingly low. Forinstance, the CNT concentration of the ink used by Song, et al. [Ref. 1]was as low as 20 μg/mL (approximately 0.002% by weight of CNTs inwater). With such a low concentration, it would take several repeatedprinting passes (overwrites) to achieve a desired CNT amount, thickness,or property; e.g., it took 8 overwrites to achieve a sheet resistivityof 20 μΩm [FIG. 5 in Ref. 1]. By contrast, with the presently inventedNGP-based ink that can carry a high NGP proportion, yet stillmaintaining a relatively low viscosity, one or two printing passes aresufficient to attain the same desired properties achieved with 5-20overwrites using CNT-based inks. This implies that the printing speed ofNGP-based inks would be much higher. This is on top of the fact thatCNTs are extremely expensive.

Currently, certain type of specialty printer can print an ink with asolution viscosity up to 150 mPa·S and some experimental printers thatare still under development can work with a viscosity up to 500 mPa·S.Even with these high-viscosity printers one would still find itdifficult, if not impossible, to print CNT-based inks with a CNT contentgreater than 0.2% by weight since their viscosity will be greater than 1Pa·S or 1,000 mPa·S. This is not the case with NGP-based inks, whichusually exhibit a much lower viscosity compared to their CNTcounterparts (with comparable additive weight or volume fractions), tobe illustrated later with examples.

Preferably, the graphene platelets have an average thickness less than10 nm and more preferably no greater than 1 nm. Preferably, theconductive ink further comprises a surfactant (or dispersing agent)and/or a binder or matrix material. The binder or matrix material may beselected from a thermoplastic, a thermoset resin, a conductive organicsubstance, a petroleum or coal tar pitch, or a combination thereof.

The conductive ink may further comprise CNTs in an amount of less than5% (further preferably less than 1%) by volume based on the totalconductive ink volume. In addition to CNTs, several other types ofconductive additives may be used to modify the properties of theconductive ink. Hence, in one preferred embodiment, the conductive inkmay further comprise a conductive additive selected from the groupconsisting of carbon nanotubes, carbon nano-fibers, carbon black, finegraphite particles, nano-scaled metal particles, and combinationsthereof.

As indicated earlier, for inkjet printing, the conductive ink shouldhave a viscosity less than 500 mPaS (<0.5 PaS). For the conductive inksintended to be screen printed or spray deposited, a much higherviscosity is acceptable. Surprisingly, the ink may contain 20% by volumeor higher of nano graphene platelets yet still exhibiting a low-shearviscosity value less than 200 PaS. This can not be attained withCNT-based inks.

Another preferred embodiment of the present invention is a conductiveink composition that, after printing onto a solid substrate to form asolid component, provides a thermal conductivity of at least 10 W/(mK),preferably at least 100 W/(mK), and most preferably at least 200 W/(mK);these are higher than thus far the highest conductivity values for CNT-or NGP-based polymer composites.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Resistivity of various printed patterns from NGP- and CNT-basedinks as a function of the number of printing passes.

FIG. 2: The viscosity of several NGP- and CNT-containing aqueousdispersions or inks over a range of shear rates.

FIG. 3: The viscosity of several inks containing NGP, CNT, CNT+NGP, andCNT+CB as conductive nano fillers.

FIG. 4: Thermal conductivity data for NGP- and CNT-based nanocompositesdeposited on a solid substrate via inkjet printing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Since the discovery of single-walled carbon nanotubes (SWNTs) andmulti-walled carbon tubes (MWNTs), a large number of potentialcommercial applications have emerged, including polymeric composites,field emission displays, electrical capacitors, and thermal managementmaterials. Although a number of different techniques have been proposedto manufacture CNT-based materials, the current demand of thesematerials is still limited due to several factors: First, the high costof CNTs at this stage of production has discouraged a wider scope ofapplication. Second, the difficulties in handling and dispersing CNTsmake their incorporation in useful matrices a challenge. Theincompatibility of CNTs with most typical solvents limits theireffective handling and widespread use, since, when placed in water ormost organic solvents, nanotubes generally quickly fall out ofsuspension even after strong sonication. Third, while only a very smallamount of CNTs may be sufficient to achieve greatly improved propertiesin some applications, the concentration CNTs used may need to be muchhigher in other applications. All these three factors have a strong,negative impact on the applications of CNTs in conductive inks.

Rather than trying to develop much lower-cost processes for CNTs, wehave worked diligently to develop alternative nano-scaled carbonmaterials that exhibit comparable properties, but can be produced inlarger quantities and at much lower costs. This development work has ledto the discovery of processes for producing individual nano-scaledgraphite planes (individual graphene sheets) and stacks of multiplenano-scaled graphene sheets, which are collectively called “nanographene plates (NGPs).” The structures of NGPs may be best visualizedby making a longitudinal scission on the single-wall or multi-wall of anano-tube along its tube axis direction and then flattening up theresulting sheet or plate. NGPs have become much lower-cost substitutesfor carbon nano-tubes or other types of nano-rods for various scientificand engineering applications. The electronic, thermal and mechanicalproperties of NGP materials have been shown to be comparable or superiorto those of carbon nano-tubes.

Direct synthesis of the NGP material had not been possible, although thematerial had been conceptually conceived and theoretically predicted tobe capable of exhibiting many novel and useful properties. Jang andHuang provided an indirect synthesis approach for preparing NGPs andrelated materials [Ref. 15]. In most of the methods for making separatedgraphene platelets, the process begins with intercalating lamellargraphite flake particles with an expandable intercalation agent(intercalant), followed by thermally expanding the intercalant toexfoliate the flake particles. In some methods, the exfoliated graphiteis then subjected to air milling, ball milling, or ultrasonication forfurther flake separation and size reduction. The NGPs prepared by usingthese methods are graphite oxide platelets since intercalation typicallyinvolves heavy oxidation of the flake graphite particles. Thermal andelectrical conductivities of these oxidized NGPs or graphite oxideplatelets are not as high as those of pristine, non-oxidized NGPs, whichcan be prepared by a direct ultrasonication method without exposinggraphite to intercalation or oxidation [Ref. 16]. It is of significanceto note that graphite is usually considered a hydrophobic material andcan not be dispersed in a polar liquid such as water. Much to oursurprise, direct ultasonication-produced NGPs, albeit being pristine,non-oxidized and non-polar graphene layers, are readily dispersable inwater and many other organic solvents, in which NGPs were producedoriginally.

The presently invented conductive inks can contain oxidized ornon-oxidized graphene platelets to meet the property requirements ofintended applications. The preparation of these two types of NGPs andtheir inks are further discussed as follows:

Using graphite as an example, the first step for the preparation ofpristine NGPs may involve preparing a laminar material powder containingfine graphite particulates (granules) or flakes, short segments ofcarbon fiber or graphite fiber, carbon or graphite whiskers, carbon orgraphitic nano-fibers, or their mixtures. The length and/or diameter ofthese graphite particles are preferably less than 0.2 mm (200 μm),further preferably less than 0.01 mm (10 μm). They can be smaller than 1μm. The graphite particles are known to typically contain micron- and/ornanometer-scaled graphite crystallites with each crystallite beingcomposed of multiple sheets of graphite plane.

The second step comprises dispersing laminar materials (e.g., graphiteor graphite oxide particles) in a liquid medium (e.g., water, alcohol,or acetone) to obtain a suspension or slurry with the particles beingsuspended in the liquid medium. The third step entails subjecting thesuspension to direct ultrasonication at a temperature typically between0° C. and 100° C. Hence, this method obviates the need or possibility toexpose the graphite material to a high-temperature, oxidizingenvironment. Preferably, a dispersing agent or surfactant is used tohelp uniformly disperse particles in the liquid medium. Mostimportantly, we have surprisingly found that the dispersing agent orsurfactant facilitates the exfoliation and separation of the laminargraphite material. Under comparable processing conditions, a graphitesample containing a surfactant usually results in much thinner plateletscompared to a sample containing no surfactant. It also takes a shorterlength of time for a surfactant-containing suspension to achieve adesired platelet dimension.

Surfactants or dispersing agents that can be used include anionicsurfactants, non-ionic surfactants, cationic surfactants, amphotericsurfactants, silicone surfactants, fluoro-surfactants, and polymericsurfactants. Particularly useful surfactants for practicing the presentinvention include DuPont's Zonyl series that entails anionic, cationic,non-ionic, and fluoro-based species. Other useful dispersing agentsinclude sodium hexametaphosphate, sodium lignosulphonate (e.g., marketedunder the trade names Vanisperse CB and Marasperse CBOS-4 fromBorregaard LignoTech), sodium sulfate, sodium phosphate, sodiumsulfonate, sodium dodecylsulfate, sodium dodecylbezenesulfonate, andTRITON-X.

Ultrasonic or shearing energy also enables the resulting platelets to bewell dispersed in the very liquid medium, producing a homogeneoussuspension. One major advantage of this approach is that exfoliation,separation, and dispersion are achieved in a single step. A monomer,oligomer, or polymer may be added to this suspension to form asuspension that is a precursor to a nanocomposite structure.

Oxidized NGPs or graphite oxide platelets may be obtained byintercalation and exfoliation of graphite. Intercalation of graphite toform a graphite intercalation compound (GIC) is well-known in the art. Awide range of intercalants have been used; e.g., (a) a solution ofsulfuric acid or sulfuric-phosphoric acid mixture, and an oxidizingagent such as hydrogen peroxide and nitric acid and (b) mixtures ofsulfuric acid, nitric acid, and manganese permanganate at variousproportions. Typical intercalation times are between one hour and fivedays. The resulting acid-intercalated graphite may be subjected torepeated washing and neutralizing steps to produce a laminar compoundthat is essentially graphite oxide. In other words, graphite oxide canbe readily produced from acid intercalation of graphite flakes.

Conventional exfoliation processes for producing graphite worms(interconnected networks of thin graphite flakes) from a graphitematerial normally include exposing a graphite intercalation compound(GIC) or oxidized graphite to a high temperature environment, mosttypically between 850 and 1,050° C. These high temperatures wereutilized with the purpose of maximizing the expansion of graphitecrystallites along the c-axis direction. In some cases, separated NGPsare readily obtained with this treatment, particularly when the graphitehas been heavily oxidized. In other cases, the exfoliated product may besubjected to a subsequent mechanical shearing treatment, such as ballmilling, air milling, rotating-blade shearing, or ultrasonication. Withthis treatment, either individual oxidized graphene planes (one-layerNGPs) or stacks of oxidized graphene planes bonded together (multi-layerNGPs) are further reduced in thickness (for multi-layer NGPs), width,and length. In addition to the thickness dimension being nano-scaled,both the length and width of these NGPs could be reduced to smaller than100 nm in size if so desired. In the thickness direction (or c-axisdirection normal to the graphene plane), there may be a small number ofgraphene planes that are still bonded together through the van derWaal's forces that commonly hold the basal planes together in naturalgraphite. Typically, there are less than 30 layers (often less than 5layers) of graphene planes, each with length and width from smaller than1 μm to 100 μm. We observed that high-energy planetary ball mills androtating blade shearing devices (e.g., Cowles) were particularlyeffective in producing nano-scaled graphene plates. Since ball millingand rotating-blade shearing are considered as mass production processes,the present method is capable of producing large quantities of NGPmaterials cost-effectively. This is in sharp contrast to the productionand purification processes of carbon nano-tubes, which are slow andexpensive.

The oxidized NGPs prepared with a rotating blade device orultrasonicator are already dispersed in a liquid, such as water,acetone, alcohol, or other organic solvent. They can be directly used asan ink or, in some cases, subjected to a further formulation procedure;e.g., removing some of the water or solvent, adding some more liquid orother ingredient (e.g., a binder or matrix resin).

It may be noted that the intercalation treatment using concentratedsulfuric-nitric mixtures have intrinsically introduced many usefulfunctional groups to the edges and surfaces of graphene layers. Thesegroups, such as hydroxyl and carbonyl, facilitate dispersion of oxidizedNGPs in a polar liquid, such as water and alcohol for the production ofconductive inks.

After extensive studies on NGP-solvent interactions, we have observedthat NGPs could be dissolved in chloroform, benzene, toluene or otherorganic solvents after oxidation and subsequent derivatization withthionylchloride and octadecylamine. Partially oxidized NGPs may alsoundergo reactions with fluorine, alkanes, diazonium salts, or ionicfunctionalization. Alternatively, we could attach soluble polymers toNGPs by various methods. For example, we could develop non-covalentassociation of NGPs with linear polymers such as polyvinyl pyrrolidoneand polystyrene sulfonate. The intimate interaction that occurs betweenthe polymer and the NGPs results in an increased dispersability of thegraphene platelets in water.

The following examples serve to illustrate the best mode practice of thepresent invention and should not be construed as limiting the scope ofthe invention:

EXAMPLE 1 Nano-Scaled Graphene Platelets (NGPs) from Natural GraphiteFlakes

A typical procedure for preparing non-oxidized NGP-based conductive inksis described as follows: Five grams of graphite flakes, ground toapproximately 10 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO surfactant from DuPont) to obtain a suspension. An ultrasonic energylevel of 95 W (Branson S450 Ultrasonicator) was used for exfoliation,separation, and size reduction for different periods of time: 0.5, 1, 2,and 3 hours. The average thickness of the NGPs prepared was found to be33 nm, 7.4 nm, 1 nm, and 0.89 nm, respectively.

For ink viscosity studies, the NGPs used were those with an averagethickness of 7.4 nm. The water content of an ink sample was adjusted byusing controlled evaporation (to increase NGP volume fraction in asuspension) or adding water (to dilute it). Suspensions with a range ofNGP volume fractions, up to greater than 45%, were prepared. Muchthinner NGPs (1 nm) were used for electrical resistivity and thermalconductivity measurements.

EXAMPLE 2 Nano-Scaled Graphene Platelets (NGPs) from Natural GraphiteFlakes (No Dispersing Agent)

Five grams of graphite flakes, ground to approximately 10 μM or less insizes, were dispersed in 1,000 mL of deionized water to obtain asuspension. An ultrasonic energy level of 95 W (Branson S450Ultrasonicator) was used for exfoliation, separation, and size reductionfor a period of 2.5 hours. The resulting NGPs, although thicker thanthose prepared with the assistance of a surfactant, were also welldispersed in water, forming a surfactant-free ink.

EXAMPLE 3 Preparation of Inks Containing Oxidized NGPs (Graphite OxidePlatelets)

Graphite oxide was prepared by oxidation of graphite flakes withsulfuric acid, nitrate, and potassium permanganate, at a ratio of 4:1:0.01 at 30° C., according to the method of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. Upon completion of the reaction, the mixturewas poured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 7. The intent was to remove all sulfuric andnitric acid residue out of graphite interstices. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debye-Scherrer X-ray technique to be approximately0.73 nm (7.3 Å), indicating that graphite has been converted intographite oxide. Graphite oxide was placed in a quartz tube, which wasthen inserted into a three-zone tube furnace pre-set at 1,050° C. andmaintained at this temperature for 60 seconds. Nitrogen was continuouslyintroduced into the quartz tube while graphite oxide was exfoliated. Theresulting graphite oxide worms were then mixed with water and subjectedto a shearing treatment using a rotating-blade device (Cowles) for 30minutes. This procedure led to the formation of oxidized NGP dispersionor ink.

EXAMPLE 4 Preparation of CNT-Based Inks (with or without a PolymerBinder)

Conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) and carboxyl functionalized single-walled carbonnanotubes (SWCNT-COOHs) were also used in the present study.

SWCNT-COOHs were purchased from Sigma-Aldrich (CNT content 90%,carboxylic acid composition 3-6 at %, bundle dimensions 4-5 nm×0.5-1.5μm). First, as a typical procedure, 2.5 mg of SWCNT-COOHs were dispersedin 20 ml ethanol by ultrasonic agitation for 20 min. Subsequently, thesolution was centrifuged at 3500 rpm for 10 min, and the supernatantsolution was separated and centrifuged again. The centrifugationprocedure was repeated until a stable dark brown solution was achieved.The concentration of SWCNT-COOHs in ethanol was found to beapproximately 0.1 g/l, corresponding to slightly less than 0.005% byvolume. Ethanol evaporation was used to adjust the CNT volume fraction.A range of aqueous/ethanol CNT dispersions were prepared for viscosityand conductivity measurements.

PEDOT-PSS (1.3 wt % in water) was purchased from Sigma-Aldrich anddiluted with DI water (polymer solution to DI water ratio of 6:4) toprepare the stock solution. The ink was made by mixing 8 ml SWCNT-COOHsolution (with a CNT concentration of 0.1 g/l) and 2 ml polymer stocksolution with vigorous stirring. A reference polymer ink solution wasmade by adding 2 ml of polymer stock solution to 8 ml of ethanol.Similarly, a reference SWCNT-COOH solution was made by mixing 2 ml of DIwater to 8 ml of SWCNT-COOH solution in ethanol.

For comparison, the inks containing NGPs prepared in Example 1 and inkscontaining oxidized NGPs prepared in Example 3 were also inkjet printed.In one case, NGP solution was mixed with PEDOT-PSS to form an ink. BothCNT- and NGP-based inks were adjusted to the maximum concentration, witha zero-shear viscosity of approximately 100 mPaS.

The printed patterns were made on high gloss photo paper using an inkjetprinter (Fast T-Jet Blazer TJB-1650 Printer from US Screen Print &Inkjet Technology with an Epson Pro 4800 Printhead) equipped with acleaned and re-filled cartridge. The printed patterns on photo paperswere lines with length and width of 40 mm and 2.5 mm, respectively. Forelectrical resistivity measurements using the 4-point probe method,Pt-electrodes (2.0×4.0 mm²) having an average thickness of ˜30 nm weresputtered on the printouts with a constant gap spacing of 10.0 mm. Theelectrical measurements were carried out using a voltmeter and a sourcemeter (Keithley 2182A Nanovoltmeter and Keithley 2400 Sourcemeter). Thestructure of the printouts was characterized by scanning electronmicroscopy and transmission electron microscopy.

The resistivity data of printed patterns are summarized in Table 1 andFIG. 1.

TABLE 1 Resistivity of various printed patterns as a function of thenumber of printing passes. No. PEDOT-PSS + PEDOT-PSS + Oxidized PrintsCNT CNT PEDOT-PSS NGPs NGP NGPs 1  7.5E01 8.50E+01 8.70E+02 2 1.00E+031.10E01 1.60E+01 1.30E+02 5 2.00E+02 4.00E+02 1.50E00 3.60E+00 7.20E+0110 1.50E+02 2.00E+03 2.70E+02 20 3.50E+01 4.20E+02 1.00E+02 30 2.00E+011.50E+02 2.60E+01

It is clear from Table 1 and FIG. 1 that, with CNT-based inks, theresistivity of printed patterns was still as high as 2,000 kΩ/squareeven after 10 repeated overwrites and 150 kΩ/square after 30 repeatedprinting passes. By contrast, NGPs provide a low resistivity of 75kΩ/square in one print. With some conducting polymer (PEDOT-PSS), theNGP-containing printed patterns also exhibit a relatively lowresistivity after just a few repeated printing passes. Further, it isclear that the oxidized NGPs are less conductive than un-oxidized NGPs.

The viscosity of NGP- and CNT-containing aqueous dispersions was alsoinvestigated and the data are summarized in FIG. 2 and FIG. 3. FIG. 2indicates that the viscosity of NGP-based inks is typically orders ofmagnitude lower than that of CNT-based ink with a comparable fillervolume fraction. The viscosities of the inks containing 1% and 3% NGPsare much lower than the viscosity of the ink containing 0.4% CNTs. Withan NGP content as high as 40%, the NGP-based ink exhibits a viscositymuch lower than that of an ink containing 9% CNTs. These are verysurprising results considering the fact that NGPs and CNTs are basicallyidentical in chemical compositions (all graphene-based), only thegeometry being different—sheet versus tube structures.

It is also highly surprising to observe that, by incorporating a smallamount of NGPs in a CNT-based ink, one can significantly reduce theviscosity of the CNT ink. This is illustrated in FIG. 3, whichdemonstrates that an ink containing 5% CNTs and 1% NGPs (totally 6% nanofilers) actually has a lower viscosity compared to an ink containing 5%CNTs only. Both samples were prepared under comparable ultrasonicationconditions. Presumably, NGPs, being two-dimensional sheets with anano-scaled third dimension, are capable of alleviating or reducing thetendency for CNTs to form aggregates and entanglements. This is animportant observation by itself since this feature enables a greateramount of CNTs to be incorporated in a matrix material, significantlybroadening the scope of CNT nanocomposite applications. In contrast, asalso shown in FIG. 3, an additional 1% of carbon black (CB) particlesactually slightly increase the viscosity of the CNT-ink.

It may be noted that an ink containing more than 40% by volume of NGPs,albeit exhibiting a relatively low viscosity, is still somewhat beyondthe suitable viscosity range of current inkjet printers. However, thesehigh-loading NGP-based inks can still be easily screen printed formicroelectronic device applications, or spray-coated (e.g., using acompressed air gun) for coating applications. Hence, the NGP-based inksalso have a utility value as coating materials that provide desiredelectrical and thermal conductivity.

Other types of fillers, other than CNTs or in combination with CNTs, canbe added to NGP-based inks to modify their properties. These includecarbon black, metal nano particles, conductive organic species, carbonnano-fibers, etc. These conductive fillers are well-known in the art.

EXAMPLE 5 Preparation of NGP- and CNT-based Inks (with a Polymer MatrixMaterial)

In order to evaluate the thermal conductivity of NGP-polymer andCNT-polymer nanocomposites obtained by inkjet printing, a series ofPEDOT-PSS/CNT and PEDOT-PSS/NGP dispersions were prepared in a waysimilar to the procedure as described above, but the polymer-to-fillerratio was varied to obtain various diluted inks that are inkjetprintable. The inks led to printed nanocomposites with various CNT orNGP contents. It may be noted that the NGPs used in this examples havean average thickness of approximately 1 nm, containing one or fewgraphene planes per platelet. They have exhibited exceptionally highthermal conductivity. The thermal conductivity data for these printednanocomposites are summarized in FIG. 4. It is clear that NGP-basednanocomposites produced from inkjet printing of corresponding conductiveinks are superior to CNT nanocomposites in terms of thermal conductivityenhancement. Furthermore, the absolute thermal conductivity values ofNGP-polymer composites reach 16 W/(mK) with only 5% NGPs. This thermalconductivity value is significantly higher than the best value thus farreported in the literature (e.g., 7 W/(mK) for 25% NGP-epoxy compositereported by Yu, et al. [Ref. 17]). With 40% of the un-oxidized NGPs, thenanocomposite exhibits an impressive thermal conductivity of 245 W/(mK).

Hence, a preferred embodiment of the present invention is a nanographene platelet-based conductive ink that is printable, comprising:(a) nano graphene platelets that have an average thickness no greaterthan 10 nm (preferably no greater than 1 nm), and (b) a liquid medium inwhich these nano graphene platelets are dispersed, wherein the nanographene platelets occupy a proportion of at least 0.001% (preferably atleast 1%, more preferably at least 3%) by volume based on the total inkvolume.

To meet the requirements of high electrical and thermal conductivity, ithas proven essential to prepare inks from non-oxidized or pristine NGPs.Hence, another highly preferred embodiment of the present invention is anano graphene platelet-based conductive ink that is printable,comprising: (a) non-oxidized or pristine nano graphene platelets, and(b) a liquid medium in which these nano graphene platelets aredispersed, wherein the nano graphene platelets occupy a proportion of atleast 0.001% by volume based on the total ink volume (preferably atleast 1%, more preferably at least 3%). Again, the inks containing highNGP loadings (e.g., >40% by volume), albeit not inkjet printable using acurrent inkjet printhead, can be applied or deposited using techniquessuch as screen printing.

The features and benefits of NGP-based conductive inks include thefollowing:

-   -   (1) NGPs are much less expensive than nano silver particles        (e.g., for RFID antenna application) and carbon nano-tubes        (CNTs).    -   (2) NGP-based nanocomposites are capable of readily forming a        thin film, paper, or coating for electromagnetic interference        (EMI) shielding and electrostatic charge dissipation (ESD)        applications.    -   (3) Due to the ultra-high thermal conductivity of NGPs (5 times        more thermally conductive, yet 4 times lower in density compared        to copper), a nanocomposite thin film, paper, or coating can be        used as a thermal management layer in a densely-packed        microelectronic device.    -   (4) A high loading of NGPs (up to 75% by wt.) can be        incorporated into a polymer matrix, as opposed to up to only        5-10% of CNTs. Both CNTs and CNFs (carbon nano-fibers) have        dispersion issues which derive from a high length-to-diameter        aspect ratio. CNTs require overcoming van der Waals forces,        while both CNTs and their larger-diameter cousins (CNFs) can        easily get entangled with one another, or “bird-nested” into        bundles which must be dispersed to optimize efficacy in many        applications. Hence, the loading of these conductive        nano-fillers increases the viscosity of the matrix resin to a        level that is not conducive to composite processing or inkjet        printing. This is not the case for the NGP-resin systems,        wherein the two-dimensional platelets can slide over one        another, leading to low resistance to shear flow even at a        relatively high NGP proportion. This feature would enable easier        application of inks/coatings (e.g., easier inkjet printing of an        NGP-containing solution) and more convenient melt processing of        polymer nanocomposites containing a high NGP loading.    -   (5) For thermal management applications, the anisotropic        properties NGPs allow them to move heat directionally, enabling        control of the heat transfer. These unique materials have an        in-plane two-dimensional thermal conductivity that can be        tailored to up to 5,300 W/mK and a through-thickness third        dimension conductivity of several W/mK. Competition materials,        such as aluminum and copper, move heat in all directions, but        due to high contact resistance, they do not transfer heat from        components efficiently. Further, NGP-based thermal interface        materials (TIM) can be easily cut and molded into intricate        shapes, sizes and thickness. More importantly, NGP-TIM can be        printed onto solid substrates that have intricate surface        profiles or difficult-to-reach spots in a microelectronic        device. In addition, NGPs can be combined with plastics, metals        or elastomers in finished components.

1. A nano graphene platelet-based conductive ink that is printable,comprising: (a) nano graphene platelets wherein each of the plateletscomprises a graphene sheet or multiple graphene sheets and the plateletshave a thickness no greater than 100 nm, and (b) a liquid medium inwhich said nano graphene platelets are dispersed, wherein said nanographene platelets occupy a proportion of at least 0.001% by volumebased on the total ink volume.
 2. The conductive ink as set forth inclaim 1 wherein the graphene platelets have an average thickness lessthan 10 nm.
 3. The conductive ink as set forth in claim 1 wherein thegraphene platelets have an average thickness no greater than 1 nm. 4.The conductive ink as set forth in claim 1 wherein the nano grapheneplatelets comprise a non-oxidized graphene platelet.
 5. The conductiveink as set forth in claim 1 further comprising a surfactant and/or abinder material.
 6. The conductive ink as set forth in claim 2 furthercomprising a surfactant and/or a binder material.
 7. The conductive inkas set forth in claim 1 further comprising a binder or matrix materialselected from a thermoplastic, a thermoset resin, a conductive organicsubstance, a petroleum or coal tar pitch, or a combination thereof. 8.The conductive ink as set forth in claim 1 wherein the grapheneplatelets occupy a proportion of at least 3% by volume based on thetotal ink volume.
 9. The conductive ink as set forth in claim 2 whereinthe graphene platelets occupy a proportion of at least 10% by volumebased on the total ink volume.
 10. The conductive ink as set forth inclaim 1 wherein the graphene platelets occupy a proportion of at least40% by volume based on the total ink volume.
 11. The conductive ink asset forth in claim 1 further comprising carbon nanotubes in an amount ofless than 5% by volume based on the total conductive ink volume.
 12. Theconductive ink as set forth in claim 1 further comprising carbonnanotubes in an amount of less than 1% by volume based on the totalconductive ink volume.
 13. The conductive ink as set forth in claim 1further comprising a conductive additive selected from the groupconsisting of carbon nanotubes, carbon nano-fibers, carbon black, finegraphite particles, nano-scaled metal particles, conductive organicspecies, and combinations thereof.
 14. The conductive ink as set forthin claim 1 wherein said ink is inkjet printable.
 15. The conductive inkas set forth in claim 1 wherein said ink is screen printable.
 16. Theconductive ink as set forth in claim 1 wherein a viscosity of said inkis less than 500 mPaS.
 17. The conductive ink as set forth in claim 1wherein a viscosity of said ink is less than 100 mPaS.
 18. Theconductive ink as set forth in claim 1 wherein a viscosity of said inkis less than 30 mPaS.
 19. The conductive ink as set forth in claim 1wherein a viscosity of said ink is less than 200 PaS and said inkcontains no less than 20% by volume of nano graphene platelets.
 20. Theconductive ink as set forth in claim 1 wherein said ink, after printingonto a solid substrate to form a solid component, provides a thermalconductivity of at least 10 W/(mK).
 21. The conductive ink as set forthin claim 1 wherein said ink, after printing onto a solid substrate toform a solid component, provides a thermal conductivity of at least 100W/(mK).
 22. The conductive ink as set forth in claim 1 wherein said ink,after printing onto a solid substrate to form a solid component,provides a thermal conductivity of at least 200 W/(mK).
 23. A nanographene platelet-based conductive ink that is printable, comprising:(a) nano graphene platelets that have an average thickness no greaterthan 10 nm, and (b) a liquid medium in which said nano grapheneplatelets are dispersed, wherein said nano graphene platelets occupy aproportion of at least 0.001% by volume based on the total ink volume.24. The conductive ink as set forth in claim 23 further comprising asurfactant and/or a binder material.
 25. The conductive ink as set forthin claim 23 wherein said ink is inkjet printable and a viscosity of saidink is less than 500 mPaS.
 26. The conductive ink as set forth in claim23 further comprising a conductive additive selected from the groupconsisting of carbon nanotubes, carbon nano-fibers, carbon black, finegraphite particles, nano-scaled metal particles, conductive organicspecies, and combinations thereof.
 27. The conductive ink as set forthin claim 23 wherein said ink, after printing onto a solid substrate toform a solid component, provides a thermal conductivity of at least 10W/(mK).
 28. A nano graphene platelet-based conductive ink that isprintable, comprising: (a) non-oxidized or pristine nano grapheneplatelets, and (b) a liquid medium in which said nano graphene plateletsare dispersed, wherein said nano graphene platelets occupy a proportionof at least 0.001% by volume based on the total ink volume.
 29. Theconductive ink as set forth in claim 28 further comprising a surfactantand/or a binder material.
 30. The conductive ink as set forth in claim28 wherein said ink is inkjet printable and a viscosity of said ink isless than 500 mPaS.
 31. The conductive ink as set forth in claim 28further comprising a conductive additive selected from the groupconsisting of carbon nanotubes, carbon nano-fibers, carbon black, finegraphite particles, nano-scaled metal particles, conductive organicspecies, and combinations thereof.
 32. The conductive ink as set forthin claim 28 wherein said ink, after printing onto a solid substrate toform a solid component, provides a thermal conductivity of at least 10W/(mK).
 33. The conductive ink as set forth in claim 28 wherein saidink, after printing onto a solid substrate to form a solid component,provides a thermal conductivity of at least 100 W/(mK).